It is known to manufacture hollow metallic articles by superplastic forming and diffusion bonding metal workpieces. These metal workpieces include elementary metal, metal alloys, intermetallic materials and metal matrix composites. At least one of metal workpieces must be capable of superplastic extensions.
In one known process of diffusion bonding the surfaces of the workpieces to be joined are cleaned. The workpieces are arranged in a stack and the edges of the workpieces are welded together, except for an aperture where a pipe is welded to the workpieces, to form a sealed assembly. The interior of the sealed assembly is evacuated and the pipe is sealed. The sealed assembly is placed in a pressure vessel and is heated and pressed to diffusion bond the workpieces together to form an integral structure. Diffusion bonding occurs when two mating surfaces are pressed together under temperature, time and pressure conditions that allow atom interchange across the interface such that the interface effectively ceases to exist.
In one known process of superplastic forming and diffusion bonding the surfaces of the workpieces to be jointed are cleaned, and at least one surface of one or more of the workpieces is coated in preselected areas with a material to prevent diffusion bonding. The workpieces are arranged in a stack and the edges of the workpieces are welded together, except for an aperture where a pipe is welded to the workpieces, to form a sealed assembly. The pipe enables a vacuum, or inert gas pressure, to be applied to the interior of the sealed assembly. The sealed assembly is placed in an autoclave and heated so as to "bake out" the binder from the material used to prevent diffusion bonding. The sealed assembly is then evacuated and the pipe is sealed. The sealed assembly is placed in a pressure vessel and is heated and pressed to diffusion bond the workpieces together to form an integral structure. Diffusion bonding occurs when two mating surfaces are pressed together under temperature, time and pressure conditions that allow atom interchange across the interface. The first pipe is removed and a second pipe is fitted to the diffusion bonded assembly at the aperture where the first pipe was located. The integral structure is located between two appropriately shaped dies and is placed within an autoclave. The integral structure and dies are heated and pressurised fluid is supplied through the second pipe into the interior of the integral structure to cause at least one of the workpieces to be superplastically formed to produce an article, or component, matching the shape of the dies.
In another known process of diffusion bonding the surfaces of the components to be joined are cleaned. The components are arranged in a stack and the stack is placed in a metal bag, the metal bag is sealed except for an aperture where a pipe is welded to the metal bag, to form a sealed assembly. The interior of the sealed assembly is evacuated and the pipe is sealed. The sealed assembly is placed in a pressure vessel and is heated and pressed to diffusion bond the components and metal bag together to form an integral structure. Diffusion bonding occurs when two mating surfaces are pressed together under temperature, time and pressure conditions that allow atom interchange across the interface such that the interface effectively ceases to exist.
In these known methods of diffusion bonding and superplastic forming it is essential that there is a vacuum in the sealed assembly in order to ensure that a satisfactory diffusion bond is formed between the workpieces, or components, and that the vacuum is maintained in the sealed assembly when they are subsequently heated and pressed together during the diffusion bonding process.
The heat and pressure applied during diffusion bonding causes the pipe to collapse, but the end of the pipe joined to the sealed assembly is supported by the joint. It is often necessary to use large internal diameter pipes, particularly for the manufacture of large articles, to provide sufficient gas conductance at reduced pressure for good evacuation of the sealed assembly prior to diffusion bonding. These large internal diameter pipes have thin walls. A problem associated with the diffusion bonding process is that the temperature and pressure applied during the diffusion bonding process is likely to cause the walls of the pipes to be sucked back into the sealed assembly because of the vacuum in the sealed assembly. If the walls of the pipe are sucked back there is a possibility that the pipe will burst and thus the vacuum in the sealed assembly will be lost. This means that diffusion bonding of the workpieces, or components, will not occur.